Dynamic Programming

# In-Depth Look

## Introduction

In Dynamic Programming (DP), a large problem is broken down into smaller subproblems, the solutions to which build on top of each other to ultimately give the solutions to subsequent subproblems. Once the entire solution space has been explored, the solution to the original problem will be achieved.

We will illustrate this method with a simple example problem.

## Example: Checkerboard Problem

Given an $n\ x\ n$ checkerboard and one checker, we want to move the checker from the top row to the bottom row. Every step, we can move the checker to either:

1. the square directly below its current location,
2. the square below and to the left (if the checker is not already in the leftmost column), or
3. the square below and to the right (if the checker is not already in the rightmost column).

Note: we will refer to the possible spaces from a square $x$ as its children.

Each time we move the checker, we receive a reward. $R(x,y)$ is the reward received when moving from square $x$ to square $y$. Rewards can be positive or negative.

The goal is to find a path starting in the top row and ending in the bottom row which maximizes the reward received, and to find this path in polynomial time.

### Brute Force Solution (CheckersBrute)

To solve this problem, we will need to explore all possible paths that can be taken from the top row to the bottom row and calculate the reward for each. This could be achieved using a brute force algorithm, as such:

1. For all squares in the top row:
• calculate the reward received by taking each valid path from that square to a square in the bottom row.
• Save each path and associate the reward with that path.
2. Return the path with the highest associated reward.

### Complexity Analysis of CheckersBrute algorithm

This algorithm looks at all possible paths originating from each of the top squares. Let’s consider a square in the middle of the top row. This square will look at the paths originating from its three children, and so on.

At level $l$, there are $3(1 + 2l)$ recursive calls made. There are $n$ rows (levels), so we have total work = \begin{align} \sum_{i=0}^n {3(1+2i)} & = \sum_{i=0}^n {3 + 6i} \\ & = \sum_{i=0}^n {3} + \sum_{i=0}^n {6i} \\ & = 3n + 6*\sum_{i=0}^n {i} \\ & = 3n + 6\frac{n(n+1)}{2} \\ & = 3n + 6*\frac{n^2 + n}{2} \\ & = 3n + 3n^2 + 3n \\ & = 3n^2 + 6n \\ \end{align} There are $n$ columns of recursive calls giving work over all columns $\leq n(3n^2 + 6n) = 3n^3 + 6n^2 = O(n^3)$.

There are two ways, however, in which this algorithm is very wasteful.

1. Many of the paths will have overlaps, but these will be recalculated for every path.
2. From any given square, there will exist at least one path which will give the highest possible reward from that square to the top row. Because of this, once the highest possible reward from a certain square has been calculated, there is no need to consider other possible paths from this square.

### Dynamic Programming Solution (CheckersDP)

Starting from the leftmost square in the top row $(0,0)$, we calculate the max path-value of its (at most) three children. If a child is not yet memoized, we recursively call the algorithm on its children. This process is repeated until the recursive algorithm is called on a bottom level square, in which case it hits the base case and returns 0.

For example, a square $a$ in the row before last will look at its three children $b_1$, $b_2$, and $b_3$. These are all base case states so they will each return a max-path-value of 0. The max $R(a,b_i)$ will be picked as $a$’s max-path-value and stored in $V(a)$: $$V(a) = \max\{R(a,b_1), R(a,b_2), R(a,b_3)\}.$$

After the call on square $(0,0)$ recurses completely, the algorithm moves onto the square to its right $(1,0)$. At this point, many of the squares along paths from square $(1,0$) are already associated with max-path-values. Because we memoize solutions to the subproblems as we go, we are able to cut out 2 of 3 recursive calls at every level (the left and middle children), reducing the amount of recursive calls to just 1.

Once the last call to the top row (n-1,0) has finished recursing, our table V will contain the max-path-value from every square on the checkerboard. To return the path to take, we simply start at the square in the top row with the highest value in V. We then look at the children of that square and pick the one with the highest value in V. We repeat this process until we reach the bottom row, in which case we are done.

• Note: to pick the last square of the path, we will have to look at the rewards for the three children of the square in the n-1th row.

### Complexity Analysis of CheckersDP algorithm

During the call to $(0,0)$, the max-path-values will be calculated for all the gray squares. In the following call to $(1,0)$, the gray squares already have values associated with them in our table V, so only the blue squares must have their values calculated.
Similarly, the calls to $(2,0)$, $(3,0)$, etc. will only calculate max-path-values for their right children and will look up the rest in V.

• Note: Even during the call to $(0,0)$, time will be saved because each value will be saved in $V$ and when other squares in the gray section need the max values for one of those, it can just query $V$. In this way, each square’s max-path-value is only calculated once.

Since we are cutting out a lot of recursive calls using memoization, we essentially only call recursively along the diagonals, which are a length $kn$. Since there are $n$ squares in the top row, we make $n$ calls of $kn$ work each. This means that the DP algorithm will have a running time of $kn*n = O(n^2)$.

• This makes sense because we only calculate the path-value of each square once, since it is memoized.

## Further Details

Because this problem involves overlapping subproblems, it is naturally a good candidate for a Dynamic Programming solution. As shown in the above analyses, by using DP instead of a brute force algorithm, we reduced the total work by a factor of $n$. In this solution, we used memoization to recursively calculate the solutions for subproblems which we then used to calculate the solutions to larger problems. Alternatively, tabulation could have been used to build up solutions from the bottom up.

Check out this page for more information on memoization and tabulation, their differences and similarities, and respective pros and cons.